Isolate houseplant from others to prevent spread of scale insects. An insecticide spray for houseplants can be used to spray both the plant and the soil surface. Because waxy shells on scale insects are impervious to insecticides, you will need to mix in some liquid soap to help insecticide penetrate beneath shells. Spray both sides of leaves and stems. If plant is heavily infested or badly damaged, it is often best to dispose of it.